Semiconductor devices, such as microelectromechanical systems (MEMS) and integrated circuits, are fabricated on semiconductor wafers by many different processing steps, sometimes as many as several hundred. These steps include deposition, etching, implantation, doping, and a variety of other processing steps.
The processing steps involved in fabricating semiconductor devices on wafers often result in stress-induced defects in the wafer, such as voids or cracks. These process-induced stress defects can reduce fabrication yield and adversely affect the reliability and performance of semiconductor devices fabricated on the wafers.
Because of the problems that can be caused by stresses induced in semiconductor wafers by fabrication processes, it highly desirable to be able to directly measure such stresses. These stress measurements can be used, for example, to identify wafers that are likely to provide low yields of semiconductor devices or which might produce devices prone to early failure.
Conventional strain gauges are generally unsuitable for directly measuring stresses built up in the wafer substrate during wafer fabrication. Electrical resistance strain gauges conventionally require a constant power supply. A constant power supply is incompatible with the conditions of wafer processing, such as etching in a corrosive wet bath and ion implantation in a high vacuum chamber.
Conventional mechanical and optical strain gauges typically include macroscopic moving parts that cannot readily be scaled down for microfabrication on wafers. Even if they can be microfabricated, their moving parts are incompatible with the conditions of wafer processing, such as high speed spinning during wafer lapping, grinding and polishing.
A need therefore exists for an integrated strain gauge that can be exposed to all of the conditions of wafer processing so that process-induced stress in wafers can be directly measured in-line. This need is particularly felt in wafer processing of MEMS products, such as inkjet printer firing units. This is because MEMS wafer processing typically involves high-stress process steps such as wafer drilling and laser ablation that are not present in conventional integrated circuit wafer processing.